This invention relates to electrostatic printing devices and more particularly to a toner delivery system for presenting toner to a charge retentive surface or to an electronically addressable printhead utilized for depositing toner in image configuration on receiver substrates.
Of the various electrostatic printing techniques, the most familiar and widely used is xerography in which a latent electrostatic image is formed on a charge retentive surface, developed by a suitable toner material to render the image visible, and the developed image is transferred to plain paper.
Another form of electrostatic printing is one known as direct electrostatic printing (DEP), in which, unlike xerography, toner is deposited directly or "written" onto a receiving surface or substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sep. 5, 1972 to Gerald L. Pressman et al.
Pressman et al. disclose an electrostatic line printer incorporating a multi-layered particle modulator or printhead including a dielectric layer sandwiched between a continuous conductive layer on one side and a segmented conductive layer on the other side. The particle modulator further includes one or more rows of apertures. Each segment of the segmented conductive layer is formed around a portion of an aperture, and is electrically isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments, while a fixed potential is applied to the continuous conductive layer. An overall applied field projects airborne charged particles through the apertures of the particle modulator, and the density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles is intercepted by a print-receiving medium to provide line-by-line scan printing. In the Pressman et al. device, toner is supplied to the control member by a uniform field which results in toner accumulations on the printhead. This disturbs the toner flow and produces irregularities in the printed image. The openings in the printhead are subject to clogging, and high speed recording is difficult.
U.S. Pat. No. 4,568,955 issued Feb. 4, 1986 to Hosoya et al. discloses apparatus to record visible images on plain paper by a developer. It includes a toner bearing developing roller spaced from and facing the plain paper. A recording electrode responsive to a signal source generates an electric field between the plain paper and developing roller, in accordance with image information, to propel toner from the developing roller to the plain paper. Mutually insulated electrodes, extending in one direction on the developer roller, are connected to A.C. and D.C. sources to produce an alternating electric field between successive electrodes and to liberate toner from the developer roll. Hosoya et al. further disclose an open-top toner reservoir below a recording electrode. A toner carrying plate in the reservoir is driven by a three phase generator to agitate the toner and produce a traveling wave that allegedly transports toner in the form of a "smoke" from the toner reservoir to the recording electrode. The use of a single traveling wave device, however, to perform all tasks (namely, charging, transport and delivery to a recording electrode) is unsuitable for recording high quality images at recording speeds of practical interest. Hosoya also does not show how to operate the traveling wave device to deliver unipolar toner, or to achieve toner motions suitable for the printing of quality images.
U.S. Pat. No. 4,814,796 issued Mar. 21, 1989 to Fred W. Schmidlin discloses a direct electrostatic printing (DEP) apparatus including a toner delivers' system in which a donor roller presents charged toner to an apertured printhead, toner being deposited on the donor roller via a magnetic brush. The donor roller is positioned adjacent to the printhead structure to form a nip area therebetween. The toner on the donor roller is excited into a cloud-like state in the nip area via an A. C. voltage applied between the donor roller and the shield electrode of the apertured printhead. In operation of the DEP apparatus, toner particles that are predominantly charged to one polarity, referred to as "right sign toner" (or RST), are passed through apertures and deposited on the receiver substrate, such as plain paper. The control electrodes which propel toner through the apertures, and an opposed paper shoe, are at voltages opposite in polarity to the charge on the RST. The voltage of the paper shoe is much greater than the voltage on the control electrodes so the RST are attracted to the paper shoe and not to the control electrodes. To prevent the passage of toner through a given aperture, its control electrode is switched to a large voltage of the same polarity as the RST. This repels the RST and forces them back toward the donor. In this circumstance no toner is deposited on the paper. The control electrode is then said to be in the OFF state.
In the OFF state, any toner in the toner cloud near the aperture which is opposite in polarity to the right sign toner, referred to as wrong sign toner (WST), will be drawn through the aperture and collected on the control electrode. The WST does not deposit on the paper because the paper shoe is the same polarity as the WST and therefore repels WST from the paper. Thus, collection of WST on the control electrode does not immediately affect image quality. It becomes a problem when an aperture is in the OFF condition for an extended duration, as needed to print large white areas. In that event, relatively large amounts of WST accumulate on the control electrodes and the electrostatic charge associated with such accumulations produces an electric field that counters the working field produced by the control voltage. Eventually, this counter field negates enough of the control field to enable right sign toner to leak through the aperture. This toner then lands on the paper, where it produces a noticeable, unwanted, gray background.
The foregoing discussion explains the fundamental reason why DEP requires the use of a magnetic brush containing a very low concentration of WST. With sufficiently low concentrations of WST in the toner supply it is possible to maintain a control electrode in the OFF state for a full page length without producing an unacceptable level of gray background. The printhead can then be restored to a clean state between pages using a cleaning process such as described in U.S. Pat. No. 4,755,837 issued Jul. 5, 1988 to Fred W. Schmidlin et al.
By way of example, a DEP apparatus designed to work with negative toner may utilize a paper shoe set to +400 volts and control electrodes biased to +50 Volts in the ON state, and -350 volts in the OFF state. In this case, the positive WST will be repelled from the paper shoe and attracted to the negative control electrode in the OFF state. With these operating voltages it is known that an 11 inch length of white, with no noticeable background, can be printed if the quantity of WST that flows to the control electrodes in the OFF state is less than 0.2% of the RST that flows to the paper in the ON state.
Another from of DEP apparatus conceived to deliver a minimum of WST to a DEP printhead is described in U.S. Pat. No. 4,743,926 issued May 10, 1988 to Fred W. Schmidlin. The toner delivery process described in that patent is based on a traveling wave toner transporting device known as a Charged Toner Conveyor (CTC). The CTC, described in detail in U.S. Pat No. 4,647,179 issued May 3, 1987 to Fred W. Schmidlin is well suited for effecting spatial separation of toner of opposite polarity while in transport on the conveyor, making it possible to extract toner of one polarity from the conveyor while leaving toner of the other polarity on the conveyor for transport to a point of use. U.S. Pat. No. 4,743,926 describes one means of extracting WST from a CTC prior to delivery to a DEP printhead. It uses a second CTC placed in face-to-face relation with the primary CTC and an electrical bias to attract WST from the primary CTC to the second CTC. The primary CTC then transports the right sign toner past the DEP printhead where it is used for printing.
Invention of the CTC was based on the idea that toner can be carried on a traveling wave in a manner analogous to the way a surfer rides water waves. Because of this analogy, the toner motion achieved on the CTC is called the "surfing mode". By analysis, it was established that at sufficiently low frequencies the toner moves synchronously with a wave while it is constantly pushed toward the conveyor surface by a normal force (perpendicular to the surface) provided by the wave itself. The toner particles move at the speed of the wave while seeking out a stable phase relation established by the average frictional drag. But in practice, the toner particles are frequently scattered off the conveyor surface by irregularities in the shape of the conveyor surface, or the shape of the toner. The scattered toner are continually returned to the conveyor surface by the normal force of the wave, producing a local toner cloud that moves synchronously with the wave. The most important aspect of this surfing mode is that toner of a given polarity ride the wave in a restricted phase range, while toner of the opposite polarity ride the wave with this phase range shifted by 180 degrees. This occurs because the wave appears inverted to a negative toner compared to the way it appears to a positive toner. The fact that the toner move spatially separated (by a half wave length) in the surfing mode makes it possible to remove one of the polarities from the conveyor with a normal force, and thereby achieve toner charge filtering. Such is the basis of my U.S. Pat. No. 4,743,926.
Another form of traveling wave toner transport device, known as an "Electric Curtain" (EC), was invented by Masuda (cf. U.S. Pat. No. 3,872,361: No. 3,778,678 and No. 3,801,869). The toner motion produced by this device, retorted to as the "curtain mode", is asynchronous, with the toner moving much slower than the wave. In the curtain mode the toner execute cycloidal like orbits (shown later), while being repelled from the conveyor surface via a force derived from the time average of the field gradient of the traveling wave in interaction with the oscillatory motion of the toner. This force is dependent on the toner moving much slower than the wave. Application of the Electric Curtain as a development means, as tacitly suggested by the aforementioned Hosoya et al., U.S. Pat. No. 4,568,955, has been frequently proposed, but not in conjunction with a toner conditioning means. Transport of toner in the curtain mode is also unsuitable for imaging applications because the toner speed is too slow to be of practical value.
I have discovered a new mode of traveling wave toner transport, which forms the basis for the present invention. This new mode is readily distinguishable from both the surfing mode and the curtain mode. It is produced by applying a uniform electric field (E.sub.b) normal to a traveling wave conveyor while operating the conveyor at a frequency sufficient to otherwise produce the curtain mode. The bias field is sufficiently large to force toner into contact with the conveyor surface, over powering the repulsive force of the wave that sustains the normal curtain mode. The toner moves slower than the wave, with periodic surges as each wave overtakes and passes through the toner. In effect, the toner attempts to catch each wave but fails because the frequency and speed of the wave is too great. Thus each wave "hunches" (lifts and thrusts forward) the toner in the direction of the wave. The motion (illustrated later) is clearly distinguishable from the surfing and curtain modes, and is referred to as the "hunching" mode. The discovery of this mode is important because the average speed of the toner can be controlled in a range that is ideally suited tier imaging applications. The average toner speed can be specifically tuned for each application via the combination of wave frequency and the strength of the bias field. Toner speeds best suited for practical imaging applications are much greater than can be achieved with the curtain mode. The desired speed range can be achieved via the surfing mode but at a lower than desired mass transport rate. Thus the hunching mode is of great practical importance, for it is the only mode capable of delivering high quantities of toner to a latent image at the optimal speed.
I discovered the hunching mode through extensive analysis of toner motions produced by traveling waves. The analytical formalism used for this investigation is described in a paper entitled "The Modes of Traveling Wave Particle Transport and their Applications" by F. Schmidlin, published in the Journal of Electrostatics, Vol. 34, 1995. This publication focuses on the previously known surfing and curtain modes. I discovered the hunching mode only recently while investigating the effect of a bias field to find a mode of toner motion more suitable for imaging applications. The discovery of the hunching mode formed the basis for the traveling wave toner conveyor systems of the present invention.
The method of design is best illustrated by examples. There are three dimensionless parameters of importance in this analysis:
1) a reduced frequency parameter, .OMEGA.=f.lambda./(bE.sub.o), where f is the frequency of the multiphase generator driving the conveyor, .lambda. is the wavelength of the traveling wave, E.sub.o is the amplitude of the electric field of the traveling wave, b=Q/6.pi..eta.a is the drift mobility of a toner having charge Q and radius a, and .eta. is the coefficient of viscosity of a particle moving in still air; PA0 2) a reduced mass parameter, M=2.pi.bE.sub.o .tau./.lambda., where .pi.=bm/Q is the viscous relaxation time for a particle of mass m; and PA0 3) a pseudo gravity parameter, G=E.sub.b /E.sub.o, where E.sub.b is the magnitude of a uniform d.c. bias field normal to the surface of the conveyor.
In previous work, the parameter G was determined by gravity, and was important only in the curtain mode of a horizontal conveyor. For the new hunching mode of this invention, gravity is negligible (as it is for the surfing mode) and G is uniquely determined by the normal bias field E.sub.b in units of E.sub.o. This force, by construction, is always directed normal to the conveyor for any orientation of the conveyor.
All possible toner motions of interest are determined by the three parameters: .OMEGA., M and G. The physical quantities which determine these parameters are given by their foregoing definitions. It should be noted that M in particular is determined by the conveyor wave-length (.lambda.), the field-amplitude (E.sub.o) of the wave, the toner charge/mass ratio (Q/m) and toner radius a. Representative values for these physical quantities are E.sub.o =3.4 volts/.mu.m; .lambda.=4 mm; Q/m.congruent.8 .mu.C/gm; and a.congruent.5 .mu.m, leading to M.congruent.40. Other choices for these parameters for imaging applications typically lead to values of M in the range between 5 and 100. Given M, the possible single particle toner motions become determined by .OMEGA. and G. These are respectively controlled by the physical operating parameters of frequency (f) and bias field (E.sub.b).
Prior traveling wave studies focused on a pure gravitational bias, G.congruent.0.01, for which the possible modes of transport are the surfing mode for .OMEGA.&lt;.OMEGA..sub.c .congruent.1.7/.sqroot.M.congruent.0.3 (for M=40), and the curtain mode for .OMEGA.&gt;.OMEGA..sub.c. The frequency .OMEGA..sub.c is the critical frequency above which the synchronous surfing mode is not possible. Characteristically, toner move at the wave speed (f.lambda.) in the surfing mode; and at a very low speed in the curtain mode--much too slow to be of practical interest in imaging applications. Representative toner trajectories for the surfing and curtain modes are shown in FIG. 8. The dimensionless coordinates (X,Z) in this figure correspond to the actual coordinates in units of .lambda./(2.pi.). The dimensionless average toner speed in the X-direction is denoted &lt;U&gt; and corresponds to the actual speed in units of bEo/.sqroot.M. FIG. 8 a shows toner catching the wave after one hop, after which the toner moves at the wave speed of 1.34, or 5 m/sec for M=5. Increasing the frequency to 0.63 causes the toner to launch into the curtain mode as shown in FIG. 8b, whence the toner slows to an average speed of&lt;U&gt;=0.0066, or 0.02 m/sec. As shown in FIG. 8c, increasing the frequency to .OMEGA.=1, causes the toner to move somewhat closer to the conveyor surface (at Z=0) at the even slower speed of &lt;U&gt;=0.0041. A graph of the average speed, &lt;U&gt;, vs. frequency, .OMEGA., for M=5 is shown In FIG. 9. Note the sharp drop in speed above the critical frequency .OMEGA..sub.c .congruent.0.61 as the mode changes from the synchronous (surfing) mode to the asynchronous (curtain) mode. For .OMEGA.&lt;.OMEGA..sub.c, the toner speed is readily adjustable with frequency. But at frequencies sufficient to produce toner mass flow rates of practical interest, the toner speed is generally too high tier quality image development. For .OMEGA.&gt;.OMEGA..sub.c, the toner move too slow for practical imaging applications.
Faced by the dilemma that no practical means of operating a traveling wave conveyor system for imaging purposes appeared possible, the idea of forcing toner close to a conveyor at high frequencies to speed up the asynchronous mode occurred to me and led to the present invention. A uniform normal force much greater than gravity can be produced by applying a DC bias field normal to the conveyor. The effect is manifest in the analysis by producing a much larger value of the "pseudo gravity" parameter G. A typical result for G=0.4 at .OMEGA.=0.75 is shown in FIG. 10. Note that the average speed, &lt;U&gt;-0.3, has increased by nearly a factor of 100 over the speed for G=0.01. Note also the significant change in character of the toner motion compared to either the curtain or surfing modes. The toner is thrust ahead (hunched) by each wave as it passes, alternately sliding in contact with the conveyor, then lifted off the conveyor by the next wave crest. When in contact with the conveyor surface, the Z-dimension is 0.07, the toner radius.
To distinguish this new mode from the others it is referred to as the "hunching" mode. The average toner speed vs. frequency for G=0.5 is compared to the average toner speed for G=0.1 in FIG. 11. Note that the higher G shifts the critical frequency (.OMEGA..sub.c) for the onset of asynchronous motion to a slightly higher value. But most importantly, toner speeds in the asynchronous hunching range are greatly increased and within the range of practical interest for imaging applications. The most usefull speed range occurs for .OMEGA. between .OMEGA..sub.c and 3.OMEGA..sub.c. This speed range is dependent on M as shown in FIG. 12. The dependence of toner speed on G for different .OMEGA. and M=5 is shown in FIG. 13. A similar family of curves is obtained for different M. As previously defined, the parameter M is predominantly determined by the conveyor wave length and toner material. In general, the useful toner speeds for imaging applications are obtained with this new hunching mode for 0.05&lt;G&lt;0.9 and .OMEGA..sub.c &lt;.OMEGA.&lt;3.sub.c. In this range, the toner movement is asynchronous and .OMEGA..sub.c is identified experimentally as the lowest frequency for asynchronous toner motion. This defines a crucial operating range claimed in the present invention. It should also be appreciated that another important attribute of the hunching mode is that the toner move in close proximity with the conveyor surface, at an average distance of &lt;Z&gt;&lt;1, as shown in FIG. 14. This feature provides the ability to deliver toner to a latent image at close range without the toner physically contacting the latent image bearing member, except in areas where the latent image, by design, attracts toner from the conveyor. This is key to obtaining a non-interactive development process. This property naturally accompanies the hunching mode when the toner are moved in the desired speed range, as defined above.
Examples showing use of the analysis to design conveyors for direct toner printing and xerographic development now follow. A conveyor of .lambda.=0.4 mm and M=40 is considered. For direct printing a speed of 15 cm/sec, or &lt;U&gt;0.11 is typically desired. By analysis this toner speed is produced by .OMEGA.=0.45, and G=0.19. For special xerographic development applications a toner speed of 50 cm/sec may be optimal. Correspondingly &lt;U&gt;=0.37 is desired, which is produced by .OMEGA.=0.38 and G=0.41. Other toner speeds suitable for different applications can be similarly found via numerical solution of the equations of motion. All possible toner motions ensue from different choices for the three dimensionless parameters M, .OMEGA. and G.
It should be appreciated that since the analysis governs single particle motion, its use is limited to the design of the conveyor system and identification of its approximate operating conditions. Actual operating parameters must be fine tuned experimentally for optimal results. Air drag caused by the collective action of large numbers of toner moving together is expected to cause an upward shift in the threshold frequency (.OMEGA..sub.c) for asynchronous motion. Compensation for this effect must be determined experimentally.
From experience and extensive analysis similar to the above I have realized that the operating conditions for a toner conveyor system which optimize the functions of loading a conveyor, charge filtering and delivery to a latent image are often incompatible. This has suggested to me the use of a segmented toner conveyor, with each segment separately optimized for its intended task. One or more segments are operated in the surfing mode for optimizing toner loading, charge filtering and general transport purposes. One segment is operated in the new hunching mode liar accepting toner form the loading or transport segment and conveying the toner past a latent image at the optimal speed. The over all performance of the conveyor system is thus improved dramatically. For certain special applications, a single conveyor operated in the new hunching mode will perform satisfactorily.
In multi-segmented conveyors it is necessary to make adjoining conveyor segments compatible. In particular, the mass flow of toner on the loading conveyor segment must be accommodated by every segment, including the delivery segment. More specifically the mass and charge per unit area transported in the slower hunching mode can not become so great that transport becomes blocked by toner pile-up on transfer from the faster surfing mode. This should not be a problem however, providing the speed reduction on transfer does not exceed the ratio of 10/1. This is because the toner coverage in the surfing mode is typically less than 10%. Phase matching of the waves on neighboring segments is unnecessary because transport on at least one of the two segments will be asynchronous. Toner transfer across the junction will be effected by toner momentum. Compatibility of operation of the different conveyor segments is therefore not a severely restrictive consideration.
The principles and analysis illustrated by the above examples can be applied to the design and operation of any segmented traveling wave conveyor system. It need only be remembered that final tuning of the operating conditions must be done experimentally. During such experimentation, a simple test to determine whether or not the toner moves synchronously with the wave is to examine the toner motion with a microscope using stroboscopic illumination. With the stroboscopic frequency at or near the wave frequency, the toner will appear in bands separated by one wavelength (or a half-wavelength with the presence of sufficient WST) when the toner particles move synchronously (as in the surfing mode). For any of the asynchronous modes, the toner will appear uniformly distributed over the conveyor, with no evidence of banding.
It is an object of the present invention to provide a means of delivering toner to a latent image with a speed and spatial distribution suitable for the format/on of high quality powder images.
Another object is to provide a segmented traveling wave toner conveyor system, with each segment operated to optimally perform its specific function. One segment loads toner onto a conveyor at a desired rate, one segment facilitates removal of toner of wrong polarity, and one segment delivers toner to a latent image at a preferred speed and spatial distribution in one embodiment of the invention, said latent image is created and transported on a photoreceptor surface, as used for xerographic copying or laser printing. In another embodiment of the invention, the latent image is created via a stationary printhead, as used in direct toner printing.
Another object is to provide a compact arrangement of components around a traveling wave toner conveyor system comprised of a loading/filtering segment and a delivery segment.
Another object is to deliver toner to a latent image bearing member (printhead, ion receptor or photoreceptor) without the use of a moving delivery member, such as a rotating donor roll, as frequently used in prior art.
Another object is to achieve a high level of toner charge purity while using a single component developer.
Still another object is to deliver toner to an image bearing member already carrying a previously developed (toned) image without disturbing (or interacting with) the previously developed toner. This so called non-interactive, or scavengeless, feature enables the formation of full color images on a single image receiver by using toner delivery systems containing different color toner in sequence, followed by only one transfer step in the cases of ionography and xerography, or no transfer step in the case of direct toner printing.
The invention, as described below, provides a new and improved means of charged toner conditioning and transport for the development of electrostatic latent images in xerography or ionography, or for delivering toner to electrostatically controlled apertures in a direct toner printing system.